1. Field of the Invention
The present invention relates to a suspension system designed to effect attitude control of a vehicle.
2. Description of the Prior Art
In suspension systems which are capable of attitude control of vehicles, for example, in active suspensions, a pressure fluid is properly supplied to and discharged from a cylinder of a suspension unit to control the level of the vehicle and suppress the change in the attitude of the vehicle caused by rolling or the like during running, i.e. travel or movement along a road surface and a pressure source is provided to supply the pressure fluid to the suspension unit.
One example of this type of conventional suspension system is shown in FIG. 1.
The prior art suspension system comprises a suspension unit 1 and a pressure source 2 for supplying a pressure fluid to the suspension unit 1. The suspension unit 1 is comprised of: a cylinder 3 which moves a wheel (not shown) relative to a car body (not shown) to thereby raise or lower the level of the vehicle; a control valve 7 which supplies and discharges the pressure fluid to and from the cylinder 3 by properly switching connections to and from two lines, that is, a supply line 4 for supplying the pressure fluid from the pressure source 2 to the cylinder 3 and a return line 5 for returning the pressure fluid from the cylinder 3 to a reservoir (tank) 6, thereby effecting attitude control of the vehicle; an accumulator 8 which is connected to the cylinder 3 to function as a spring element; and a damping force generating mechanism 9 which is provided between the cylinder 3 and the accumulator 8 to generate damping force. The control valve 7 is a solenoid-operated three-port and three-position switching valve which is arranged such that, at the position .circle.a , the pressure fluid is supplied to the cylinder 3 from the pressure source 2 to raise the level of the vehicle; at the position .circle.b , the pressure fluid is returned from the cylinder 3 to the reservoir 6 to lower the level of the vehicle; and at the position .circle.c , which is a neutral position, neither supply nor discharge of the pressure fluid is carried out. It should be noted that the amount of pressure fluid which is to be supplied to or discharged from the cylinder 3 is determined by the time during which the control valve 7 is open or the degree of opening of the valve 7.
The pressure source 2 is comprised of: a fixed displacement pump 11 which is activated in response to the rotation of an engine E of the vehicle to dischage under pressure fluid drawn from the reservoir 6 through a line 10; an accumulator 13 which is provided along a line 12 which connects together the fixed displacement pump 11 and the supply line 4 of the suspension unit 1 to accumulate the pressure fluid; a check valve 14 for preventing the pressure fluid from flowing backward from the accumulator 13 toward the fixed displacement pump 11; and a control mechanism 15 which effects constant flow control to supply the pressure fluid at a predetermined flow rate indenpendently of the rotational speed of the engine E and which also effects unloading control to return the pressure fluid discharged from the fixed displacement pump 1 to the reservoir 6 when the pressure of the pressure fluid supplied to the suspension unit 1 has reached a predetermined value.
The constant flow control that is effected by the control mechanism 15 is realized by a mechanism which comprises a restrictor 16 provided along the line 12 and a two-port and two-position flow control valve 18 provided in a line 17 which connects together the line 10 and the side of the restrictor 16 which is closer to the fixed displacement pump 11 so that, when the difference between the pressures of the pressure fluid at the upstream and downstream sides of the restrictor 16 exceeds a predetermined value, the flow control valve 18 is activated to return a part of the pressure fluid discharged from the fixed displacement pump 11 to the line 10.
The unloading control is effected by means of a two-port and two-position switching valve 20 provided in a line 19 which connects together the line 12 and the return line 5 in such a manner that, when the pressure of the pressure fluid between the check valve 14 and the suspension unit 1 has reached a predetermined value, the switching valve 20 is switched so as to return the pressure fluid discharged from the fixed displacement pump 11 to the reservoir 6.
It should be noted that the reference symbol R in FIG. 1 denotes a relief valve which is provided to return the pressure fluid to the line 10 when the pressure of the pressure fluid rises abnormally because, for example, of a breakage of the switching valve 20 that effects the unloading control. The reference symbol C in FIG. 1 denotes a cooling pipe which is provided to cool the pressure fluid which has been raised in temperature by being compressed in the oil hydraulic circuit.
The following is a description of the operation of the above-described conventional suspension system.
As the engine E starts rotating, the fixed displacement pump 11 is activated to suck the fluid from the reservoir 6 and discharge it toward the suspension unit 1, and the pressure fluid is accumulated in the accumulator 13. As the rotational speed of the engine E increases, the flow rate of the pressure fluid discharged from the fixed displacement pump 11 increases correspondingly and the difference between the pressures of the pressure fluid at the upstream and downstream sides of the restrictor 16 also increases. When the pressure difference exceeds a predetermined value, the flow control valve 18 is activated to return a part of the pressure fluid discharged from the fixed displacement pump 11 to the line 10 through the line 17, thus allowing the pressure fluid to be supplied to the suspension unit 1 only at a predetermined flow rate. When the pressure of the pressure fluid supplied to the suspension unit 1 has reached a predetermined value, the switching valve 20 is switched so as to pass the pressure fluid to the return line 5 through the line 19, thus effecting unloading control.
The above-described conventional suspension system suffers, however, from the following problems.
The fixed displacement pump 11 continuously repeats the suction, compression and discharge of the fluid from the starting of the engine E, and the discharge of the pump 11 increases in proportion to the rotational speed of the engine E. On the other hand, the amount of oil which is to be supplied to and discharged from the suspension unit 1, that is, the amount of oil consumed, is not directly related to the rotational speed of the engine E but is limited within a predetermined range, and the displacement volume of the fixed displacement pump 11 is set so that the amount of oil required can be ensured near the lowest speed of the engine E. Therefore, when the engine E rotates at high speed, the discharge of the fixed displacement pump 11 also increases in proportion to the rotational speed of the engine E and the pump 11 repeats suction, compression and discharge of the fluid in excess of the amount of oil required, thus imposing a heavy load on the engine E. In addition, an excess of the oil discharged is returned to the line 10 by the constant flow control effected by the flow control valve 18, which means that the fixed displacement pump 11 performs wasteful work.
In the unloading control, the oil that is discharged from the fixed displacement pump 11 is returned to the reservoir 6 through the switching valve 20 and the line 19 and therefore the pump 11 produces substantially no load. For this reason, when the fixed displacement pump 11 repeats the ordinary compressing operation and the unloading control operation while the engine E is rotating at high speed, the load applied to the engine E varies by a large margin, resulting in a shock every time the compressing operation and the unloading control operation are switched over from the one to the other, and thus causing an uncomfortable ride for the driver.